Nothing's actually wrong with the Mario Party series--as a Mario Fan you'd love it if you had at least one. I just don't see the reason to pay for a new one, when they are all pretty much the same game. Yeah, new mini games, but that's not a huuuuge reason.
We're running low on the totally AWESOME games. M&L:PiT was one of those games. SM64DS is the first DS game a person should own. But if you look at TMK's GameCube list, you'll see that the only platformers are SMSu (no one's favortie) LM (too short), Wario World (I didn't hear any outstandings for it, either), and PMTTYD (next to SSBM, the only GameCube game I can't live without).
BUT, as we've seen through a certain member (banned), too much of the old games is also annoying. You need a little of everything, not all of one thing. I have Mario Party 4 and don't regret buying it. It's fun to play, especially when you have 3 human players. But games such as SMB or SMW are necessities to your Mario game collection. Have a nice balance.